This invention relates to melt-processable fluoropolymers having improved stress crack resistance and smooth surfaces after extrusion.
Copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are commonly referred to as fluorinated ethylene-propylene (FEP) resins. The HFP content usually ranges between about 10 and 20 weight percent (wt %). The FEP resins are melt-processable, having a melt viscosity adequate for conventional melt processing. However, FEP does not exhibit the high temperature properties and thermal stability of PTFE. Depending on their HFP content, these resins have a crystalline melting point from about 250 to 270xc2x0 C., and have a maximum use temperature of around 200xc2x0 C. Typical applications include wire insulation and molded parts.
Copolymers of TFE with perfluoro (propyl vinyl ether) (PPVE) are commonly called perfluoroalkoxy (PFA) resins. The PPVE content usually ranges between about 2 and 10 wt %. The PFA resins generally have better thermal stability and better mechanical properties at high temperatures when compared to FEP resins. The PFA resins are also melt-processable with a crystalline melting point from about 300 to 310xc2x0 C., and they have properties similar to PTFE. Typical applications include expansion joints and liners for pipes and fittings, tubing, and film. Equipment made from PFA is used extensively in the semiconductor industry for demanding chemical handling applications.
Briefly, the present invention provides a fluorothermoplastic composition comprising interpolymerized units derived from about 94 to about 97.5 mole percent (mol %) tetrafluoroethylene (TFE), about 2 to about 3 mol % perfluoro (propyl vinyl ether) (PPVE), and about 0.5 to about 3 mol % hexafluoropropylene (HFP). The fluorothermoplastic composition has a number of double flexure cycles to failure (flex life cycles) that fits the equation:
log(flex life cycles)xe2x89xa70.71+4.0*(MFI (xe2x88x920.294)), 
wherein MFI is melt flow index in grams per 10 minutes and is described below.
In another aspect, the present invention provides a fluorothermoplastic composition comprising interpolymerized units derived from about 94 to about 97 mol % tetrafluoroethylene, about 0.75 to about 3 mol % perfluoro (propyl vinyl ether), and about 1.5 to about 3.5 mol % hexafluoropropylene, which also has a minimum flex life according to the equation above.
The present invention also provides a method of improving stress crack resistance. The invention further provides a method of making a fluorothermoplastic composition comprising the steps of (a) providing comonomers in proportions selected from (i) from about 94 to about 97.5 mol % TFE, from about 2 to about 3 mol % PPVE, and from about 0.5 to about 3 mol % HFP, or (ii) from about 94 to about 97 mol % TFE, from about 0.75 to about 3 mol % PPVE, and from about 1.5 to about 3.5 mol % HFP, and (b) polymerizing the comonomers to produce a fluorothermoplastic, then (c) shaping an article from the fluorothermoplastic, wherein the fluoropolymer has a melt flow index of less than about 25 g/10 min and a Stress Crack Resistance of greater than 24 hours and/or the fluoropolymer has a number of double flexure cycles to failure that fits the equation: log(flex life cycles) greater than 0.71+4.0*(MFI(xe2x88x920.294)).
The present invention also provides fluorothermoplastic articles, such as tubing, tube fittings, film, and coatings.
It is an advantage of the present invention to provide a melt-processable fluorothermoplastic composition useful in applications requiring high chemical resistance and high stress crack resistance.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The following description more particularly exemplifies certain preferred embodiments utilizing the principles disclosed herein.
The present inventors have recognized that the stress crack resistance property has particular importance. Prior methods of improving the stress crack resistance of tetrafluoroethylene-based plastics involved the addition of perfluoropropylvinyl ether (PPVE) with the problems associated with this method, e.g., difficulty of incorporation during polymerization, multiple melting points over a broad temperature range in the resultant fluoropolymer, higher cost fluoropolymer. The present invention solves these problems and improves the stress crack resistance property of fluoropolymers.
The present invention provides a fluorothermoplastic composition. This composition can be represented as a modification of polymers classified in the art as PFA fluorothermoplastics, which are generally copolymers of TFE with a small amount of PPVE. The fluorothermoplastic composition of the present invention comprises particular ratios of interpolymerized units derived from a combination of TFE, PPVE, and HFP.
Surprisingly, the fluorothermoplastic compositions of the present invention have higher performance at lower levels of PPVE, as compared to known materials. Such higher performance is shown in test results, such as stress crack resistance and flex life.
In the present invention, at least three fluorinated monomers are copolymerized. The primary component is tetrafluoroethylene (TFE), which comprises at least about 90 mole percent (mol %), more preferably at least 94 mol %, of the interpolymerized units in the inventive fluorothermoplastic composition.
At least two other fluorinated monomers are interpolymerized with the TFE. These include perfluoro (propyl vinyl ether) (PPVE), and hexafluoropropylene (HFP). The level of PPVE is from about 0.5 to about 5 mol %, more preferably from about 0.75 to about 3 mol %. The level of HFP is from about 0.25 to about 5 mol %, more preferably from about 0.5 to about 3.5 mol %.
As used herein, the sum of the mol % of each of the first three components described totals 100. When another material is interpolymerized with these three comonomers, it is preferably present at levels below about 10 mol %, more preferably below about 5 mol %, and even more preferably below about 2 mol %, of the total composition including the first three comonomers and any additional material. In such an embodiment, the mol % levels of TFE, PPVE, and HFP described herein are relative to each other and do not include a fourth or other material.
In a particular embodiment, the interpolymerized units are comprised of about 94 to about 97.5 mol % TFE, about 2 to about 3 mol % PPVE, and about 0.5 to about 3 mol % HFP. In another particular embodiment, the interpolymerized units are comprised of about 94 to about 97 mol % TFE, about 0.75 to about 3 mol % PPVE, and about 1.5 to about 3.5 mol % HFP.
In another particular embodiment, the interpolymerized units are comprised of from about 94 to about 97.5 mol % TFE, above about 2 (more preferably above about 2.1 and in some instances above 2.5) to about 3 mol % PPVE, and from about 0.5 to about 3 mol % HFP. In another particular embodiment, the interpolymerized units are comprised of from about 94 to about 97 mol % TFE, from about 0.75 to about 3 mol % PPVE, and from about 1.5 (more preferably from about 1.9, and even more preferably from about 2.2) to about 3.5 mol % HFP. Minor amounts of additional fluorinated monomers and/or non-fluorinated monomers may be interpolymerized with the compositions described above without departing from the scope of the invention. Other monomers include, for example, perfluorovinyl ethers such as:
CF2xe2x95x90CFOCF3, CF2xe2x95x90CFOCF2CF3, CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2CF3,
CF2xe2x95x90CFOCF2CF2CF2OCF3, CF2xe2x95x90CFOCF2CF(CF3)OCF2CF(CF3)OCF2CF2CF3, and
CF2xe2x95x90CFOCF2CF2OCF3.
For some fluoropolymer applications, other monomers particularly those not fully fluorinated are avoided, such as in semiconductor applications where high purity and/or thermal stability is needed. Thus, the fluorothermoplastic composition of the invention may consist essentially of the particular comonomers in the ratios described above.
The melting point of TFE-based fluorothermoplastics decreases as the level of interpolymerized monomers is increased. As the level of interpolymerized monomers is increased further, more than one melt peak may become evident by differential scanning calorimetry. Often the additional melt peaks occur at temperatures above the desired melt peak temperature. One feature of the present invention is the absence of any significant melt peak above 300xc2x0 C., as measured by differential scanning calorimetry (DSC) according to ASTM D 4591. This means that the sum of the areas under a DSC curve (scan rate of 10xc2x0 C. per minute) for any peaks above 300xc2x0 C. total less than about 10%, more preferably less than about 5%, and even more preferably about 0% of the composition. The invention allows compositions having melt peaks below 300xc2x0 C., which can have end-of-melting points above 300xc2x0 C.
In most compositions of the present invention, there are no melt peaks above 300xc2x0 C. Thus, the fluorothermoplastics of the present invention have their major melt peak at a temperature below about 300xc2x0 C. or even below about 290xc2x0 C., and above about 250xc2x0 C., more preferably above about 270xc2x0 C.
One important property of the inventive fluoropolymer compositions is a smooth surface. Comparatively rough surfaces may trap impurities or provide sites for undesirable biological growth during the useful life of a fluorothermoplastic article containing a fluorothermoplastic composition. Surface roughness can be measured by optical interferometry using commercially available instruments. One such instrument is available as Wyko Optical Profiler Model NT330, available from Veeco, Inc., Tucson, Ariz. A mean level is calculated for a sample surface and the deviation from this mean over the surface area sampled is determined. The average of the absolute deviation from the mean is reported as absolute roughness, Ra, in nanometers. The fluorothermoplastic compositions of the present invention typically have Ra levels below about 150 nm, more preferably below about 125 nm, and even more preferably below about 100 nm. The molecular weight of the composition and the rate of cooling after melt processing can affect surface roughness. Thus, increasing molecular weight or cooling a given composition at a higher rate can minimize spherulite size and reduce surface roughness.
Another important property of the inventive fluoropolymer compositions is stress crack resistance. This property can be evaluated by bending fluoropolymer samples around a small radius and then exposing them to a stress cracking agent in an environment known to cause stress cracking.
Stress cracking agents are those materials known to contribute to stress cracking. Such agents include, for example, alkanes such as iso-octane, lower alcohols such as methanol, and surfactants such as fluorinated surfactants and fluids including surfactants. The activity of these agents may be accelerated with increasing temperature and increasing concentration.
The environment known to cause stress cracking may be the actual conditions to which a final article may be exposed. The environment may be made more severe for accelerated testing purposes, such as by increasing temperature, using a more concentrated or more aggressive agent, and/or increasing the stress applied to the article. The materials of the present invention may be compared with known materials in an accelerated manner by bending samples around a small radius, such as a 5 mm; using solvent such as iso-octane; and using an elevated temperature, such as above about 70xc2x0 C. The duration of the exposure (in hours) until cracking is reported in hours.
Molecular weight also has been used as an indicator of stress crack resistance, with increasing molecular weight typically providing increasing stress crack resistance. Melt flow index (MFI) correlates with molecular weight and is easily determined. Thus, two materials having a similar MFI can be compared as having similar molecular weight. In the present invention, materials having different compositions can be evaluated at high MFI (low molecular weight) as another form of accelerated testing, which may be used in combination with one or more other acceleration factors. Typical conditions for melt flow testing include a temperature of 372xc2x0 C. and a 5 kg weight, with further details described in ASTM D-1238. The weight (grams) of material extruded in 10 minutes is reported.
Particular embodiments of the fluorothermoplastic compositions of the present invention have an MFI below about 10 g/10 min., more preferably below about 8 g/10 min., and even more preferably below about 5 g/10 min.
The present invention provides fluorothermoplastic polymers having a Stress Crack Resistance (conditions of 3 mm sample thickness, 5 mm radius bend, iso-octane at 80xc2x0 C. are used herein as xe2x80x9cSCRxe2x80x9d) of greater than 24 hours when the melt flow index of the fluorothermoplastic composition is less than about 25 g/10 min. More preferably, the fluorothermoplastic composition having a melt flow index of less than about 15 g/10 min also has an SCR of greater than 7 days. Even more preferably, the fluorothermoplastic composition having a melt flow index of less than about 5 g/10 min also has an SCR of greater than 30 days.
Increasing flexural fatigue strength (xe2x80x9cFlex Lifexe2x80x9d) also correlates with increasing stress crack resistance. In this test, a film strip is weighted and then flexed through an angle of 90xc2x0 at a frequency of around 250 double flexures (back and forth) per minute until the strip breaks. The number of cycles until break are recorded as the flex life. More detailed information can be found in ASTM D-2176.
The present inventors have discovered that the materials of the present invention, at a particular composition, show increasing flex life as MFI decreases according to the equation:
log(flex life cycles)xe2x89xa70.71+4.0*(MFI(xe2x88x920.294)). 
In some embodiments, the flex life is higher such that the y-intercept of this equation is also higher, such as 1.0 or even 1.5, rather than 0.71.
Thus, when a particular composition shows promising results in the accelerated testing of SCR and/or Flex Life, the MFI of that material can be decreased to further improve stress crack resistance in the intended end use of the fluoropolymer material. The MFI of each material compared should match to remove the possible performance premium due to lower MFI.
In addition to the parameters described above, the level of reactive end groups may be reduced to improve processing and/or provide other benefits. Generally, a level of reactive end groups below about 150 such end groups per million carbon atoms in the fluoropolymer is preferred (also described as 150 ppm). When desired, the level of reactive end groups can be reduced through methods known in the art to levels of 10 ppm or even lower. In some applications, such as where higher thermal stability and/or higher purity is preferable, low levels are desired.
The present invention also provides a method of improving stress crack resistance which involves the steps of providing a fluorothermoplastic composition as described above, shaping an article from that composition, and exposing the article to a stress cracking agent.
The present invention also provides a method of making a fluorothermoplastic composition comprising the steps of (a) providing comonomers of TFE, HFP and PPVE, then (b) polymerizing the comonomers to produce a fluorothermoplastic in proportions selected from (i) from about 94 to about 97.5 mol % TFE, from about 2 to about 3 mol % PPVE, and from about 0.5 to about 3 mol % HFP; or (ii) from about 94 to about 97 mol % TFE, from about 0.75 to about 3 mol % PPVE, and from about 1.5 to about 3.5 mol % HFP. This is followed by the step of shaping an article from the fluorothermoplastic, such as by extruding through a die to coat a surface or wire, form film or tubing, or extruding into a mold to produce a desired shape. The fluoropolymer thus produced has a melt flow index of less than about 25 g/10 min and a Stress Crack Resistance of greater than 24 hours and/or a number of double flexure cycles to failure that fits the equation:
log(flex life cycles)xe2x89xa70.71+4.0*(MFI(xe2x88x920.294)). 
The fluoropolymer thus produced may also have a Stress Crack Resistance (3 mm thick sample, 5 mm radius bend, iso-octane, 80xc2x0 C.) greater than a comparative fluorothermoplastic composition having a similar melt flow index. This fluorothermoplastic composition may also have a Stress Crack Resistance greater than a comparative fluorothermoplastic composition having a similar melt flow index and a higher level of PPVE.
Additives such as carbon black, stabilizers, plasticizers, pigments, lubricants, fillers, and processing aids typically utilized in fluoropolymer compounding can be incorporated into the compositions.
This invention is useful in semiconductor applications where smooth surfaces, high purity, and resistance to environmental stress cracking are desirable properties. The invention is useful in shaped articles, such as tubing, tube fittings, films, and coatings.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.